The invention relates to water based resin compositions containing biopolymer, e.g. from starch, in particular in the form of biopolymer nanoparticles having good storage stability. The resins are intended for the production of coatings with fair properties such as adhesion, barrier, solvent and wet resistance, mechanical strength, applicability, durability and film formation when applied to various types of substrates. Along with the introduction of a renewable resource, the presence of a considerable amount of biopolymer leads to an enhanced biodegradable character of the coating.
Various types of polymer coatings containing biopolymers, such as polysaccharides and proteins, are known. For example, starch is utilised in a wide variety of coating applications, whether or not in combination with other polymer resins. For some end-users, it is highly desirable to provide starch compositions in a ready-to-use and coatable form.
Starch dispersions refer to aqueous starch systems where the granular starch structure has partly or totally been disrupted using sufficient work or heat. In excess of water, the process of starch hydration and granule destruction is known as gelatinisation and can be facilitated or impeded by chemical modifications (cf. converted, derivatized or cross-linked starch) (xe2x80x9cStarch:Chemsitry and Technologyxe2x80x9d, R. L. Whistler, J. N. BeMiller, E. F. Paschall Eds., Academic Press, London, 1984). Pregelatinisation and physical treatment (e.g. via extrusion, drum- or spray drying) of starch (and derivatives) yields cold-water dispersible systems. Although there is no need for cooking, pregelatinised starch still requires substantial mechanical energy for dispersion in water. The stability of many starch dispersions over time is an additional problem since the amylose fraction, and to a much lesser extent also the amylopectin fraction, shows a strong tendency towards retrogradation, a process of crystallisation resulting in gel formation or precipitation. U.S. Pat. No. 5,032,683 describes the production of a stable aqueous starch composition having a coatable viscosity, wherein the starch has been gelatinised and reacted at temperatures above 70xc2x0 C. in the presence of a blocked glyoxal compound. However, converted (thin-boiling) and/or chemically modified starch are the advised starch materials for these compositions. The subject matter of U.S. Pat. No. 5,116,890 comprises the preparation of water-dispersible, self cross-link lattices based on a starch hydrolyzate to produce stable emulsions with low viscosity. The process requires undesirable chemicals and is quite laborious.
Blends of starch and synthetic polymers obtained via thermoplastic processing, are also well-known in the art (xe2x80x9cProduction of Thermo-Bioplastics and Fibers based mainly on Biological Materialsxe2x80x9d, H. G. Fritz, T. Seidenstxc3xccker, U. Bxc3x6lz and M. Juza, EU-Study AGRE-0200-DE, 1994). These materials are usually developed for the production of moulded or shaped articles, not for the preparation of latex compositions for coating purposes. Numerous patents describe the destructuration of starch in combination with hydrophilic and hydrophobic synthetic components, such as vinyl alcohol copolymers (EP 327505, EP 408503), polyvinylesters and polyesters (EP 327505, US 5439953), aliphatic polyesters (WO 92/19680), polylolefines (WO 92/20740), ethylene acrylic acid (EAA) copolymer (US 4133784, US 5262458). In U.S. Pat. No. 5,262,458, it is noted that in the starch/EAA blend after processing part of the destructurized starch ( less than 30% of total starch in the mixture) is found as particles with diameter lower than 1 micrometer.
Recently, a new thermo-mechanical process has been proposed for the production of cross-linked biopolymer material, in particular starch (EP 99200203 patent application). The thermo-mechanical process uses an extrusion treatment at raised temperatures and under conditions of high shear and high pressure. A plasticiser is preferably present during the thermo-mechanical treatment. Most importantly, the process is conducted in the presence of a cross-linking agent. The obtained starch material can readily be dispersed in cold water at increased solids content (up to 40 wt. %), and with relatively low viscosity (typically lower than 100 mPa.s at 25xc2x0 C. and 10 wt. %) as compared to other cold-water dispersible starches. The size of the dispersed starch particles is in the submicron size range, typically less than 200 nm on the average.
However, the limited storage stability of these colloidal starch dispersions makes them unsuitable for practical application. Depending on the nature of the starch used, shelf life may be as low as a few hours at 20 wt. % before the system turns irreversibly to a gel. Moreover, the sensitivity to humidity of starch is often a restrictive factor for coating application where wet resistance, durability, strength, barrier, etc. are required.
It has now been found that the addition of certain synthetic polymers to the colloidal starch dispersions of EP 99200203 patent application not only imparts desired properties to the coating but surprisingly, also stabilises the aqueous formulation over time. This constitutes the main object of the present invention.
It was found that hydrophilic resins and/or resins with a hydrophilic/hydrophobic character can, surprisingly, stabilise these nanoparticles in water. Upon storage, the compositions exhibit constant particle size and viscosity for at least 1 day up to one month or even more.
The present invention therefore provides a water based polymer composition comprising a biopolymer and a synthetic polymer resin, characterised in that the biopolymer results from a mechanical thermoplastic processing of a polysaccharide and/or protein starting material using shear forces in the presence of a crosslinking agent, and the synthetic polymer resin consists of a water based hydrophilic resin and/or hydrophilic/hydrophobic resin.
According to the invention, the water based polymer resin is in the form of a solution, or a dispersion, or an emulsion or a colloid.
According to the invention, the polymer resin can be an anionic, cationic and/or nonionic modified resin.
According to the invention, the polymer resin is composed of a water based resin which has been selected from polyurethanes, polyesters, polyethers, polyesterurethanes, polyacrylates, polyvinylacrylates, polystyreneacrylates, styrene-butadiene, poly(meth)acrylic acid, polyvinylalcohols, polyvinylacetates, polyvinylethers, polyethylenevinylacetates, polyethylenevinylalcohols, epoxy resin, alkyds, epoxy-, urethane-, polyester-, amino-, and amido-(meth)acrylates, or mixtures thereof.
Preferably, the polymer resin is chosen from polyurethanes, polyester-urethanes, epoxy-acrylates, polyester-acrylates, urethane-acrylates, polyacrylates, polystyrene-acrylates, styrene-butadiene and epoxy resin or mixtures thereof.
According to this particular embodiment, the said polymer resin in the composition may comprise functional groups such as hydroxyl, methylol, carbonyl, carboxyl, sulfonyl, amino, epoxy, acetyl acetoxy, (meth)acrylic and/or vinylic groups.
According to EP 99200203 patent application, in the process for producing the biopolymer nanoparticles, the biopolymer is subjected to thermoplastic processing using shear forces, a crosslinking agent being added during the mechanical treatment.
Thermoplastic processing as used in this context means a thermomechanical treatment, which is in particular an extrusion treatment performed at elevated temperature (above 40xc2x0 C., especially up to 140xc2x0 C.) under conditions of high shear and high pressure (e.g. between 5 and 150 bar). The shear can be effected by applying at least 500 J of specific mechanical energy (SME) per g of biopolymer. The elevated temperature can be moderated, in case of starch, by using an alkaline medium or by using pregelatinised starch. During the thermomechanical treatment, the biopolymer is present in high concentration, especially a concentration of at least 40, more preferably at least 50 wt. %, in an aqueous solvent, such as water or a water/alcohol mixture.
It is preferred that a plasticiser is present, such as a polyol (ethyleneglycol, propyleneglycol, polyglycols, glycerol, sugar alcohols, urea, citric acid esters, etc.) at a level of 5-40% by weight of the biopolymer. A lubricant, such as lecithin, other phospholipids or monoglycerids, may also be present, e.g. at al level of 0.5-2.5% by weight.
An essential step in the process is the crosslinking during the thermomechanical treatment. Conventional crosslinkers such as epichlorhydrin and other epoxides, dialdehydes (e.g. glutaraldehyde, glyoxal), triphosphates, divinyl sulphone, can be used for polysaccharide biopolymers, while dialdehydes, thiol reagents and the like may be used for proteinaceous biopolymers. Glyoxal is a particularly suitable crosslinker. The crosslinking reaction may be acid- or base catalysed. The level of crosslinking agent can conveniently be between 0.1 and 10 wt. % with respect to the biopolymer.
After said thermomechanical treatment, the biopolymer can readily be dissolved or dispersed in an aqueous medium to a concentration between 4 and 40 wt. %. This results in starch nanoparticles, which are characterised by an average particle size of less than 200 nm.
The biopolymer material may result from a mechanical thermoplastic processing of biopolymers selected from polysaccharides, such as starch, cellulose, hemicellulose, gums and/or from cereal, vegetable or animal proteins such as wheat gluten, whey protein, gelatin and mixtures and/or modified derivatives thereof.
Preferably, the biopolymer is native or modified starch (e.g. starch-ethers, starch-esters, oxidized starch) from tuber or cereal origin, preferably potato, waxy maize, tapioca or rice.
According to the invention, 2-90%, preferably 2-50% and more preferably 2-30% by weight of the total dry polymer resins is biopolymer.
Colloidal dispersion of cross-linked biopolymer can be added to the water based polymer resin, or conversely the latter can be added to the biopolymer nanoparticle dispersion. Alternatively, the extruded cross-linked biopolymer material can be mixed directly into the aqueous polymer resin.
According to the invention, this may result in a water-based polymer resin composition containing bio- and synthetic polymers, and having a solids content of 5-40% by weight, preferably of 5-30 wt. % and more preferably of 5-20 wt. %. The dry resins may contain up to 90%, preferably up to 50% and more preferably up to 30% by weight of biopolymer material.
According to the invention, these biopolymer and synthetic polymer resin containing compositions may be used as oxygen-barrier coating for paper, plastic or cellophane films and packaging, when combined with copolymers of vinylalcohol. In combination with water-based acrylate, styrene-acrylate, styrene-butadiene and vinyl-acetate resins, they may be used for paper and board-upgrading (i.e. adjustment of water-vapour transmission and printability, recycling, compostability). Water-based polyester/starch systems find potential application as textile sizing agent. In combination with epoxy and polyurethane resins, these compositions are respectively applicable as anti-fouling paints, e.g. in marine applications, and wood coatings (primers, lacquers) with fair properties (hardness and mechanical resistance) for parquet and furniture. 1K and 2K systems can be designed which are curable by radiation or oxidation, acid catalysis (with melamine or urea based crosslinkers), or using isocyanate crosslinkers. Stable formulations with pigments and filling material can be developed for water-based inks and paints as well.
Destabilisation of dispersions can occur according to various mechanisms, such as sedimentation (or creaming), coagulation/coascervation (causing flocculation or gelation) and Ostwald ripening. Obviously, extrusion modified starch (EMS) dispersions are subjected to irreversible gelation upon storage, which is quite common for cold-water dispersible starch dispersions (often due to the presence of amylose). This results in an unusable product for coating applications. It can be stated that a mixture of EMS with a polymer resin shows an improved stability if the gelation process is slowed down or impeded with respect to the behaviour of pure EMS. Hence, the composition can be handled over a longer period. In this sense, improved stability of aqueous EMS dispersions is achieved.
The addition of a second synthetic polymer resin to a stable binary EMS/synthetic polymer mixture does not affect the stability negatively, e.g. a composition of poly-acrylate, polyurethane and EMS where at least one synthetic compound independently improves the stability of EMS. Similarly, pigments typically used for water-based inks or paints can be added to a stable binary mixture. It is worth mentioning the reasonable performances of the ink compositions. At typical concentration levels, additives commonly used in paints do not affect the stability negatively in comparison to the individual binary mixture. This is why the present invention also concerns paint and ink formulations based on the polymers compositions described in this specification.
In order to estimate the stability in time of the water based polymer compositions, three methods were utilised, i.e.,
a) Visual inspection
The behaviour of the aqueous preparation as a function of time is evaluated qualitatively by visual inspection.
b) Particle size determination by Dynamic Light Scattering (DLS).
For many preparations, dynamic light scattering is a convenient tool to follow the time-evolution of the average size and the size distribution of the dispersed particles in the composition. This was used as a quantitative indicator of the dispersion stability. The light scattering measurements were performed at 25xc2x0 C. on a MALVERN Autosizer lo-C instrument equipped with a 8-bit correlator. As long as a tolerable scattered light signal could be measured, samples were diluted with demineralised water to a concentration lower than 0.1 wt. %. At higher concentrations, it was verified that multiple scattering and concentration (interparticle interactions) effects were insignificant for the estimated particle sizes. The time-correlation function of the scattered light intensity was recorded at an angle of 90xc2x0 (usually in triplicate). The intensity correlation data were analysed with the so-called xe2x80x9cCONTINxe2x80x9d software package (S. W. Provencher, Comput. Phys. Commun. 27, 1982) to assess particle size distributions and the average particle size. As diameter estimation from DLS data is inherently intensity-weighted (i.e. weighted to the sixth power of the diameter), it enables to detect the formation of large-sized entities in time. When multimodal size distributions were observed, the average particle diameters of the separate modes were calculated independently.
c) Rheology measurements
Rheological profiles were typically recorded over a shear rate range of 4 decades [0.01-100 sxe2x88x921] by means of a Paar-Physica UDS200 rheometer using a Zldin measuring device in a Couette type configuration. The evolution of the rheological profiles was followed over time to determine the stability range.
The features and specific aspects of the invention will become apparent from the following description of specific examples of the invention. It should be observed that the specific aspects of these examples are only set forth as preferred embodiments of what is meant in the context of the above general disclosure of the invention. These examples should not be interpreted as a limitation of the scope of the invention as such and as expressed in the claims at the end of this specification.